With the recent trend of super-miniaturization in semiconductor integrated circuits, it has been suggested that reduction in device yield arises from the presence of crystal originated particle (hereinafter referred to as “COP”), microdefects of oxygen precipitates that become the nuclei of oxidation induced stacking fault (hereinafter referred to as “OISF”), interstitial-type large dislocation (hereinafter referred to as “L/D”), and the like.
COP is a pit originated from a crystal that appears on a wafer surface when the silicon wafer that has undergone a mirror polishing is subjected a SC-1 rinse with the use of a mixed solution of ammonia and hydrogen peroxide. When the wafer is measured with a particle counter, the pit is detected as a particle (light point defect, LPD). The COP becomes a cause of deteriorating electrical characteristics such as time dependent dielectric breakdown (TDDB) characteristic and time zero dielectric breakdown (TZDB) characteristic of oxide films. In addition, COP existing on a wafer surface can create a height difference in a wiring process of devices, which can become a cause of wire breakage. In addition, it becomes a cause of leakage or the like in the element-isolating portions, lowering product yield.
It is considered that OISF originates from micro oxygen precipitates formed during crystal growth, which form the nuclei thereof; and it is a stacking fault that is exposed during a thermal oxidation process or the like in the manufacture of semiconductor devices. This OISF becomes a cause of faults in devices, such as an increase in leakage current. L/D is also called dislocation cluster, or dislocation pit, because a silicon wafer having this defect forms an etching pit having an orientation when it is immersed in a selective etchant solution including hydrofluoric acid as a main component. This L/D also becomes a cause of deteriorating electrical characteristics, such as leakage characteristics, isolation characteristics, and the like.
For the reasons stated above, it has been necessary to reduce COP, OISF and L/D defects in silicon wafers used in the manufacture of semiconductor integrated circuits.
U.S. Pat. No. 6,045,610 and the corresponding Japanese Unexamined Patent Publication No. 11-1393 discloses a defect-free ingot that does not have these COP, OISF, and L/D, and a silicon wafer sliced from the ingot. This defect-free ingot is an ingot composed of a perfect region [P], where [P] is a perfect region in which neither agglomerates of vacancy-type point defects nor agglomerates of interstitial silicon-type point defects are detected in an ingot. The perfect region [P] exists between a region [V] and a region [I] in an ingot; in the region [V], vacancy-type point defects are predominant and defects in which supersaturated vacancies are agglomerated are contained, whereas in the region [I], interstitial silicon-type point defects are predominant and defects in which supersaturated interstitial silicons are agglomerated are contained.
Japanese Unexamined Patent Publication No. 2001-102385 shows that the perfect region [P], which does not have defects in which point defects are agglomerated, is classified into a region [Pv] in which vacancy-type point defects are predominant and a region [Pi] in which interstitial silicon-type point defects are predominant. The region [Pv] is a region that is adjacent to the region [V] and has a concentration of vacancy-type point defects that is less than the minimum concentration of vacancy-type point defects at which an OISF nucleus can be formed. The region [Pi] is a region that is adjacent to the region [I] and has a concentration of interstitial silicon-type point defects less than the minimum concentration of interstitial silicon-type point defects at which an interstitial-type large dislocation can be formed.
The ingot composed of the perfect region [P] is produced within the range of a V/G ratio (mm2/minute·° C.) such that OISF (P band) generated in a ring-like shape during a thermal oxidation treatment disappears from the central area of the wafer and that L/D (B band) does not occur, where the pull rate of the ingot is V (mm/minute) and the temperature gradient with respect to the ingot's axial direction in the vicinity of the solid-liquid interface between the silicon melt and the silicon ingot is G (° C./mm).
Conventionally, the following method has been adopted in order to measure a distribution of secondary defects generated in an ingot by a heat treatment, that is, agglomerated defects, over the axial direction and over the diametric directions of the ingot. First, an ingot is sliced in the axial direction to prepare samples. Then, these samples are subjected to a mirror etching, are then heat-treated at 800° C. for 4 hours in a nitrogen or oxidizing atmosphere, and are subsequently further heat-treated at 1000° C. for 16 hours. The heat-treated samples are measured with the use of such methods as copper decoration, Secco-etching, X-ray topography analysis, and lifetime measurement. Generally, the density of the oxygen precipitates formed in a sample by a heat treatment is substantially proportionate to the concentration of oxygen. When the concentration of oxygen dissolved in an ingot is less than 1.2×1018 atoms/cm3 and more than 9.0×1017 atoms/cm3, oxygen precipitates appear at a high density in the ingot due to the heat treatment and therefore the foregoing methods are capable of clearly distinguishing a region [V], a P band, a region [Pv], a region [Pi], a B band, and a region [I].
However, when the concentration of oxygen dissolved in an ingot is low and less than 1.0×1017 atoms/cm3, such as in the case of the ingot that is pulled with a magnetic field being applied according to an MCZ (magnetic-field-applied CZ) method, the density of the oxygen precipitates that occur due to the heat treatment is not sufficiently high. For this reason, the above-described conventional method suffers from a problem in that those regions cannot be distinguished clearly. For example, when recombination lifetimes are measured subsequent to a heat treatment, the difference between the measurement value of the recombination lifetime in the region [Pv] and the measurement value of the recombination lifetime in the region [Pi] becomes smaller in a sample having a low oxygen concentration than in a sample having high oxygen concentration. For this reason, there has been a drawback in that the boundary between the region [Pv] and the region [Pi] cannot be distinguished clearly with samples having low oxygen concentrations. Moreover, depending on conditions of the heat treatment, the boundary between the region [Pv] and the region [Pi] shifts toward either the region [Pv] side or the region [Pi] side. This is attributed to the fact that the density and size of the oxygen precipitates change in the sample depending on heat treatment conditions. For this reason, it has been difficult to measure real point defect regions at high precision. In addition, when the concentration of oxygen dissolved in the ingot is not more than 9.0×1017 atoms/cm3, it has been impossible to clearly distinguish the boundary between the region [Pv] and the region [Pi], and moreover, it has been difficult to clearly measure the P band, which corresponds to the boundary between the region [V] and the region [Pv], and the B band, which corresponds to the boundary between the region [Pi] and the region [I].
Furthermore, when the concentration of oxygen dissolved in the ingot is high, 1.2×1018 atoms/cm3 or higher, the density of the oxygen precipitates that appear due to the heat treatment is too high; therefore, with the above-described conventional methods, it has been difficult to precisely measure the P band, which corresponds to the boundary between the region [V] and the region [Pv].
It is a first object of the present invention to provide a method of easily identifying the region [Pv] and the region [Pi] in an ingot as well as the boundaries thereof even when the concentration of oxygen contained in the ingot is low.
It is a second object of the present invention to provide a method of identifying a defect distribution in a silicon single crystal ingot, by which the boundary between the region [Pv] and the region [V] in an ingot can be easily identified even when the concentration of oxygen dissolved in the ingot is high.
It is a third object of the present invention to provide a method of identifying defect distribution in a silicon single crystal ingot that easily identifies both the boundary between the region [V] and the region [Pv] and the boundary between the region [Pi] and the region [I] in the ingot even when the concentration of oxygen dissolved in the ingot is low.